The invention concerns a pneumatic actuator system and a method for controlling such a system. The invention can be applied to any mechanism including a pneumatic actuator. For instance, vehicles use often pneumatic actuators for parking brakes or service brakes.
Considering the example of a truck, the actuation system is open, which means that the ambient air is drawn in an air inlet pipe, compressed by a compressor and sent to the different actuators of the vehicle brake by means of an air management system. The actuators use the compressed air during a braking operation and reject it, via an exhaust line, after use. On most systems, the air discharged by actuators is rejected directly in the atmosphere and the compressor is supplied, at each cycle, with air at atmospheric pressure. Commonly the air management system of a brake system is defined to work between 5 and 12.5 bars. This high pressure level leads to high power demand of the compressor when working with ambient air.
A basic solution is to use a supercharged compressor, that is a compressor using air that is already compressed. In that way, the power required by the compressor is less important than the power needed when using ambient air.
Two concepts are known to create a supercharged compressor. A first concept consists in or comprises adding a turbine on the air inlet line of the compressor. However, this implies a supplementary cost and adds packaging constraints. A second concept is to take air directly in the engine air inlet manifold. In this way, there is no need for extra devices to divert the air flowing in the engine, but the air consumption of the compressor disrupts the engine combustion tuning.
Another solution is to create a closed loop in the pneumatic actuator system, which means that the air discharged by the actuator is collected and put back into circulation in the pneumatic actuator system. The air discharged by the actuator being at a pressure superior to the atmospheric pressure, the energy consumed by the compressor is reduced.
Such a pneumatic actuator system is disclosed in DE-A-10 2008 045 713. This pneumatic actuator system is designed so that the compressed air discharged by the actuator, for instance service brake actuators, is directed towards a pneumatic cylinder. This pneumatic cylinder comprises a first chamber for receiving discharged air and a second chamber separated from the first chamber by a piston. A spring tends to push the piston so that the volume of the first chamber is compressed. Therefore, the pressure of the air discharged by the different actuators pushes the piston against the elastic action of the spring. Then, the volume of the first chamber increases and the pressure diminishes consequently. The air accumulated in the first chamber of the pneumatic cylinder is used afterwards by the compressor. The major drawback of this system is that, in case of an important brake operation, a large volume of air at high pressure would be rejected by the actuators and the expansion of the first chamber, receiving this volume would not decrease the pressure under a satisfactory pressure for the actuators. Therefore, the air flowing in the system might be over-pressurized so that the actuators can't work properly. More precisely, actuators are traditionally of the “simple effect” type, during a working phase, the pressure applied at the inlet makes it change from a release position to a working position. After this working phase, the pressure at the inlet must decrease during an exhaust phase to allow the actuator to come back to its release position. In the pneumatic actuator system of DE-A-10 2008 045 713, the pressure at the inlet of the actuator can remain too important during the exhaust phase. Consequently, the capacity of the actuator to come back to its release position can be limited.
GB-A-2 344 389 discloses a pneumatic actuator system working with a servo device. The system comprises a compressor, means for drawing atmospheric air in the system, and an air tank divided into a low pressure compartment and a high pressure compartment. The servo device is activated by an effort on the pedal brake of the vehicle. In operating conditions, the compressor draws air from the low pressure compartment, compresses it and rejects it in the high pressure compartment. The actuator assembly has a housing defining a working chamber, which is supplied with compressed air coining from the high pressure compartment. The air discharged by the actuators flows then to the low pressure compartment, which implies that the pressure inside the low pressure compartment increases. Here again, in case of a hard braking operation, the air flowing in the system might be over pressurized.
An aspect of the present invention involves a pneumatic actuator system functioning in closed loop, while preventing an overpressure to occur in the system.
To this end, the invention concerns a pneumatic actuator system, comprising at least one pneumatic actuator, a compressor, providing compressed air to the pneumatic actuator, a first intake to supply the compressor with external air, and a second intake connected, via a recirculation line, to an exhaust line of the pneumatic actuator and arranged to supply the compressor with air discharged from the pneumatic actuator.
According to the invention, the pneumatic actuator system also includes a selecting valve which is arranged on the exhaust line of the pneumatic actuator, between the pneumatic actuator and the compressor, this selecting valve is switchable between a recirculation position, where the air flow is directed to the recirculation line and an exhaust position where the air flow is directed to a low pressure circuit, depending at least on pressure of air in the recirculation line downstream from said selecting valve or in an air tank of the pneumatic actuator system that is arranged downstream from the selecting valve on the recirculation line and wherein air discharged from the pneumatic actuator can accumulate.
Thanks to the invention, the pneumatic actuator system can function in closed loop without risking an overpressure of the air flowing in the system. Indeed, in the recirculation position, the valve directs the air flow exhausted by the actuator back to the compressor while, in the exhaust position, the air discharged by the actuator is vented to the low pressure circuit. In this way, only a sufficient amount of compressed air exhausted by the actuator is reused, the remaining amount of air being vented to the low pressure circuit. Therefore, given that the amount of air put back into circulation is controlled, there is limited risk to disrupt the functioning of the pneumatic actuator.
According to further aspects of the invention which are advantageous but not compulsory, such a pneumatic actuator system may incorporate one or several of the following features:                The low pressure circuit is opened to the atmosphere or is connected to the circuit of a low pressure system.        The air tank is arranged between the selecting valve and the second intake.        The selecting valve switches in the exhaust position when the pressure of air in the air tank, or in the recirculation line. Increases to a pressure threshold value.        The system may further comprise at least one pressure sensor to measure pressure of the air in the recirculation line downstream from said selecting valve or in the air tank (16).        Alternatively, the system can comprise at least two pressure sensors:                    a first one to measure downstream from the selecting valve in the recirculation line or preferably in the air tank, a first air pressure and            a second one to measure upstream of the selecting valve, preferably in an outlet of the pneumatic actuator, in the exhaust line of the pneumatic actuator or in the pneumatic actuator, for instance in a working chamber or in an exhaust chamber of the pneumatic actuator, a second air pressure.                        In this case the system may comprise an electric unit or electronic control unit configured to compare the electric or electronic signals generated by the first sensor depending on the first air pressure and by the second sensors depending on the second air pressure and configured to generate a new signal depending on the result of the comparison to control the position of the selecting valve.        The selecting valve can switch in the exhaust position when the first air pressure is balanced with the second air pressure.        Alternatively, the selecting valve switches in the exhaust position when the difference between the first air pressure and the second air pressure is inferior to a threshold value.        The system includes another valve, which allows switching between external air and air discharged from the pneumatic actuator in order to supply the compressor.        The system includes several pneumatic actuators and an air management system which carries out the repartition of air between the different pneumatic actuators of the system.        
Besides, another subject matter of the invention is a method for controlling a pneumatic actuator system of a vehicle wherein said pneumatic actuator system comprises:
at least one pneumatic actuator,
a compressor, providing compressed air to the pneumatic actuator,                a first intake, to supply the compressor with external air,        a second intake, connected to an exhaust line of the pneumatic actuator, via a recirculation line, and arranged to supply the compressor with air discharged from the pneumatic actuator, and        as selecting valve which is arranged on the exhaust line of the pneumatic actuator, between the pneumatic actuator and the compressor, and        
which is able to switch between a recirculation position, where the air flow is directed to the recirculation line, and an exhaust position where the air flow is directed to a low pressure circuit,
Said method comprises the steps of                detecting at least one pressure of air between the pneumatic actuator and the second intake,        switching the selecting valve between said recirculation position and said exhaust position depending on said at least one pressure of air.        
Preferably, the step of detecting at least one air pressure between the pneumatic actuator and the second intake comprises the detection of a pressure of air in the recirculation line downstream from the selecting valve or in an air tank that is arranged on the recirculation line downstream from the selecting valve and wherein the air discharged from the pneumatic actuator can accumulate.
Preferably, in the step of switching the selecting valve between said recirculation position and said exhaust position depending on said at least one pressure of air, the selecting valve switches in the exhaust position when the air pressure in the air tank or in the recirculation line, increases to a pressure threshold value.
Alternatively, the step of detecting at least one air pressure between the pneumatic actuator and the second intake comprises the detection of a first pressure of air in the recirculation line downstream from the selecting valve or preferably the detection of a first pressure of air in an air tank arranged on the recirculation line downstream from the selecting valve, and also comprises the detection of a second pressure of air upstream from said selecting valve preferably in an outlet of the pneumatic actuator, in the exhaust line of the pneumatic actuator or in the pneumatic actuator, for instance in a working chamber or in an exhaust chamber of the pneumatic actuator.
Following the step of detecting at least one pressure of air between the pneumatic actuator and the second intake, the method may further comprise a step of comparing the first and the second pressures of air.
In an implementation of the method, in the step of switching the selecting valve between said recirculation position and said exhaust position depending on said at least one pressure of air, the selecting valve switches in the exhaust position when the first pressure of air is balanced with the second pressure of air.
In another implementation, in the step of switching the selecting valve between said recirculation position and said exhaust position depending on said at least one pressure of air, the selecting valve switches in the exhaust position when the difference between the first pressure of air and the second pressure of air is inferior to a threshold value.